MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 513 



given an account of the longitudinal fission in the common Actinian, Metridiwin marginatum 

 Milne Edwards, and shows that the asexually formed polyps <lo not reproduce nil the features 

 characteristic of the sexually produced individuals. In some of the specimens examined, Parker 

 found that fission of the stomodseal tube was still incomplete, the organ being Y-shaped, a single 

 inner end opening into the gastro-vascular cavity, and the two outer ends opening each through 

 a distinct mouth on a single disk. Generally in the fission specimens each mouth was mono- 

 glyphic (provided with only one gonidial groove), instead of diglyphic, as in normal forms; and 

 with tin 1 monoglvphie condition was associated only one pair of directives. No evidence was 

 forthcoming as to the formation of new siphonoglyps or new directives in fission polyps, while 

 there were practically twice as many non-directive mesenteries in double specimens as in single 

 ones. In any given case the assumed plane of division passed through either two primary 

 exocoeles or two entocceles, never a primary entoccele on one side and a primary exoccele on the 

 other. The production of regular hexamerous diglyphic specimens by non-sexual methods was 

 not observed: such specimens were found to number about one-fifth of the total collected, and 

 are with good reason assumed to be the products of sexual reproduction. 



The West Indian stichodactylinous anemones, Actinotryx sancti- Thomas Duch. and Mich., 

 and Ricordea jforidn Duch. and Mich., also reproduce by discal tission, and frequently more than 

 one oral aperture is present within a single tentacular zone. In the latter species as many as 

 seven mouths have been found on a single disk, thus recalling such a coral form as Manicina. 

 Professor McMurrich's (1889a) anatomical studies of these two species, and also mine (1900), have 

 shown that the mesenteries in both species are irregularly arranged, and in some polyps no 

 directives occur, while in others only one pair is present. 



The results on polypal fission in corals may be thus summarized: 



1. The larval polyps of fissiparous species develop for a time like other hexamerous species. 

 Before the introduction of tission the mesenteries are regularly arranged in two or more alter- 

 nating hexameral cycles, and two pairs of directives are present. 



■I. The first fission plane passes through the entoccele of two lateral mesenterial pairs, 

 approximately at right angles to the directive plane and longer oral axis, and divides the 

 stomodaHim and the mesenteries attached to it into practically equal halves, so that only one 

 pair of directives is inserted on each stomodieal tube. 



3. For a time the products of simple fission continue their development according to the 

 regular cyclic plan, but before long fission is repeated, and each stomodseum and the mesenteries 

 associated with it may he again divided into equal halves, or one part may lie larger than the 

 other. In probably every division the fission plane is included within two opposite entocceles. 

 At an early stage in the development of fission polyps growth may proceed more rapidly at one 

 region than at another, and thus introduce irregularities in the cyclic plan. 



1. Beyond the two primary pairs no new directive mesenteries are ever introduced, so that in 

 any fissiparous colony, however large, only the protocnemic directives occur, situated widely 

 apart, at what may be regarded as the two morphological extremities of the colony. 



With such results before one, it is clear that care must be exercised in attempting to establish 

 relationships from the absence of one or both pairs of directives in mature polyps of both corals 

 and Actinians. The history of the individual polyp must be taken into accouut before such an 

 occurrence can he regarded as a specific peculiarity. As a general rule, one would be justified 

 in assuming asexual reproduction by tission for polyps with only one or no directives, especially 

 if accompanied by irregularities in mesenterial growth. 



KISSK >N IX PORITKS. 



Polyps displaying a bioral disk are very rare on colonies of Porites. After an examination 

 of scores of living colonies of all the West Indian species, only a single example exhibiting this 

 condition has been found, although polyps showing an increase of tentacles beyond the usual 

 twelve are by no means scarce: from these all stages in the development of the mesenteries 

 beyond the primary six pairs are readily secured. These proportions present a marked contrast 



